At present, as optical recording media, binary optical recording media constructed so that binary data can be recorded by forming pits by irradiation with a recording laser beam and the binary data can be reproduced based on the presence/absence of such pits are in widespread use. Due to demands in recent years for improvements to the recording density of optical recording media, research has been conducted into a recording method that adjusts the focused beam diameter of a recording laser beam to record recording data at a high density.
On the other hand, aside from methods that adjust the focused beam diameter, the development of multi-level optical recording media where one out of a plurality of marks with different meanings can be recorded in each virtual recording cell is also progressing. With such multi-level optical recording media, a characteristic is used whereby, for example, an altered part (hereinafter also referred to as a “recording mark”) where the light transmissivity is lowered appears in one part of a virtual recording cell that is being recorded upon on the optical recording medium due to an irradiated amount of the recording laser beam being switched between several levels, and the proportion of an entire virtual recording cell taken up by a recording mark varies according to the irradiated amount of the recording laser beam. That is, when a reproduction laser beam is irradiated onto this multi-level optical recording medium, an effect is caused by the light transmissivity in virtual recording cells in which the above-described recording marks have been formed, resulting in there being several levels (for example, five levels) of light reflectivity for the reproduction laser beam. Accordingly, by associating the respective light reflectivity levels with respective data contents, it is possible to record one out of a plurality of data in one virtual recording cell. Here, the expression “light transmissivity” refers to the proportion of the reproduction laser beam irradiated onto a virtual recording cell that passes through the virtual recording cell, and the expression “light reflectivity” refers to the proportion of the reproduction laser beam irradiated onto a virtual recording cell that passes through the virtual recording cell, is reflected off a reflective layer of the multi-level optical recording medium, and then passes back through the virtual recording cell and exits to the periphery of the multi-level optical recording medium.
When reproducing recording data recorded on this multi-level optical recording medium (hereinafter this is also referred to as “multi-level reproduction”), the recording data is reproduced by specifying the light reflectivity of a virtual recording cell irradiated with the reproduction laser beam as a first, second, third, etc., level out of a plurality of light reflectivity levels. Here, in order to record recording data (recording marks M) at an even higher density than a normal binary optical recording medium, the size of the virtual recording marks S on a multi-level optical recording medium is set so as to be smaller than the pits on a binary optical recording medium. Also, the reading speed (i.e., read number of recording marks) per unit reproduction time should be set higher for a multi-level optical recording medium than a normal binary optical recording medium and the standard rotational speed (that is, a linear velocity when recording and reproduction are carried out at 1× speed) is set higher than the standard rotational speed of a normal binary optical recording medium. This means that during multi-level reproduction, in the case where the irradiation power of a reproduction laser beam is set at a similar level as the irradiation power (in this case, 0.7 mW or below) of the reproduction laser beam set for a normal binary optical recording medium, the ratio (“C/N”, also referred to below as “SNR”) of a noise component included in the reflected light increases, which can lead to a decrease in the read accuracy for recording data and to the occurrence of tracking errors when the medium is rotated at high speed. Accordingly, the present inventors tried to solve the above problems by increasing the irradiation power of the reproduction laser beam during multi-level reproduction.